1. Field of the Invention
The present invention relates to a thermal transfer sheet.
2. Description of the Related Art
A thermal transfer system by using a sublimation dye transfers a large number of color dots to a transfer receiver through a very short time heating so as to reproduce a full color image based on the color dots of a plurality of colors.
In this thermal transfer system, a so-called sublimation thermal transfer sheet, in which a dye layer composed of a sublimation dye and a binder is disposed on one surface of a base material sheet, e.g., a polyester film, is used as a thermal transfer sheet.
In the thermal transfer system, a thermal transfer sheet is heated from the back with a thermal head in accordance with image information so as to transfer a dye contained in the dye layer to a transfer receiver (photographic paper) and, thereby, form an image.
At this time, regarding the thermal transfer sheet, it is desired that a surface on the side coming into contact with the thermal head stably exhibits low friction over low density image printing to high density image printing. In general, the thermal transfer sheet is provided with a heat-resistant lubricating layer on the surface opposite to the surface, on which the dye layer is disposed, in order to prevent fusion with the thermal head and give smooth running smoothness.
As for the heat-resistant lubricating layer, a method, in which a thermosetting resin layer is formed so as to give heat resistance to a base material film, is used frequently. The abrasion resistance and the heat resistance are given to the thermal transfer sheet by this method. Although a good heat-resistant lubricating layer is obtained by using this method, it is difficult to give a slip property by merely a binder, and it is necessary to conduct heat curing. Consequently, the process becomes complicated and, as a result, a large production time is taken.
As for methods for solving such problems, a method, in which a polyamide imide silicone is used (refer to Japanese Unexamined Patent Application Publication No. 08-244369, for example), and a method, in which a mixture of a polyamide imide and a polyamide imide silicone is used as a binder (refer to Japanese Unexamined Patent Application Publication No. 2001-334760, for example), have been proposed.
In the case where the above-described binder is used, the transition temperatures Tg of the polyamide imide and the polyamide imide silicone are 200° C. or higher and, therefore, the heat resistance is given to the thermal transfer sheet. Furthermore, the slip property is given by an action of a silicone unit of the polyamide imide silicone and, thereby, a good heat-resistant lubricating layer is obtained without conducting heat curing.
However, in the case where merely the above-described polyamide imide and polyamide imide silicone are used as the binder, a polyimide raw material is used and, thereby, the cost becomes high. Moreover, the polyamide imide and the polyamide imide silicone have poor adhesion to a base material and, therefore, powder falling may occurs, so that the coating film stability is insufficient.
In addition, the coating film is whitened easily in drying and, therefore, it is necessary to pay attention to the drying condition.
Furthermore, a lubricant or a filler may be added to the heat-resistant lubricating layer when the slip property is adjusted and the thermal head polishing property is given. In the case where they are kept dispersing, there is a disadvantage in using merely the polyamide imide having no adsorption group nor active site, and coagulation may occur easily because of insufficient dispersibility.